Resonators are critical components in microwave and radio frequency circuits. As used herein, these frequencies include HF, defined as 100-10 m (about 3-30 MHz); VHF, defined as 10-1 m (about 30-300 MHz); UHF, defined as 1 m-10 cm (about 300 MHz to about 3 GHz); and microwave, defined as about 300 MHz to about 300 GHz. (See, e.g., A. B. Carlson, Communication Systems, An Introduction to Signals and Noise in Electrical Communication, p. 8, McGraw-Hill, N.Y., 1986; O. P. Gandhi, Microwave Engineering and Applications, p. 1, Pergamon Press, N.Y., 1981.) One of the many types of resonators is a helical resonator, i.e. a cavity resonator consisting of a helical coil in a shield. A helical resonator operates as a slow-wave transmission line with a relatively high characteristic impedance. In practice, a helical resonator is preferred as the resonance element in bandpass and bandrejection filters of a center frequency from several tens of MHz to about 2 GHz. They are used extensively as preselection filters in receivers, as interstage filters in IF amplifiers and as resonant elements in oscillators. Additionally, they have been simple to fabricate from metals, have good quality factors ("Q"), are relatively small in size, and exhibit high sensitivity (i.e., high signal-to-noise ratio).
The small size and compact shape of such helical resonators can be significant advantages in some applications. For example, their compact size and high Q qualify these resonators to serve as bandpass filters operating at 1 GHz in mobile communications systems. Mobile radios, satellites, and cellular telephones have very tight specifications on adjacent channel noise performance. As channel spacings get closer together, further improvement in performance is required to reduce interference and noise level between adjacent channels. In principle, a high Q bandpass filter including high T.sub.c superconducting helical resonator coils should outperform those that use metal coils.
Two intrinsic properties of superconducting materials, low surface resistance and frequency-independent penetration depth, make high critical temperature (T.sub.c) oxides potentially ideal materials for passive microwave devices. Low surface resistance should correspond to low loss and higher quality factor, "Q", in microwave system components. A frequency-independent penetration depth means that ceramic superconductors, in theory, should introduce no dispersion into a microwave device even at very high frequencies. Consequently, existing circuits should be improved by replacing the usual metal coils with low loss, high T.sub.c superconducting ceramic coils. Resulting devices should have less insertion loss, high Q, and much smaller circuits.
However, the dielectrics of substrates and geometry of devices may still produce losses. In reality the quality of superconducting materials dominates the performance of passive microwave devices. Recently, high T.sub.c materials of microwave quality in the form of thin films epitaxially grown on low dielectric loss substrates have been developed that lessen or eliminate such dielectric and geometric losses. Numerous research activities are being pursued to take advantage of the properties of such newly developed high T.sub.c materials. Development of such thin film devices has advanced much further than that of helical devices, at least in part because high quality films are more easily produced than high quality bulk materials. The operating properties, for example the surface resistance, R.sub.s, of helical resonator coils and their microwave penetration depth, .lambda., appear to be related to the quality of the coil materials.
European Patent Application No. 183453 describes production of helical resonators using a bulk YBa.sub.2 Cu.sub.3 O.sub.7-x fabricated by a viscous processing method. These resonators, which operate at 77K and cover the frequency range from 0.1 to 2.0 GHz, outperform their copper counterparts by a factor of 5 in unloaded Q. (1/Q.sub.u =1/Q.sub.1 +1/Q.sub.c, where Q.sub.u is unloaded Q, Q.sub.1 is loaded Q, and Q.sub.c is the Q of the copper shield.) In the process described in EP 183453, the powder is mixed with a viscous polymer solution at a solids fraction of 0.52. The resulting mixture is processed on a two roll mill until a homogeneous mixture forms. Wires or rods are shaped by extrusion through a series of dies ranging from 0.2-4 mm diameter, and are dried at 80.degree. C. The binder is removed and the wires are densified.
The process described in EP 183453 involves many steps, and elaborate precautions must be taken to prevent cracking during the drying step. As a result, this process is time consuming and is impractical for large scale production. It would be advantageous to find a more reliable and efficient process for producing high T.sub.c superconducting ceramic helical resonator coils.